As known, in civil air transport, airlines generally try to improve their operational procedures, more particularly, so as to:                reduce the fuel consumption and the emission of greenhouse gas,        reduce noise in urban areas; and        improve the regularity of the travel times, even with highly impaired weather conditions (wind, low clouds, bad approach visibility).        
Such improvements could be achieved through performance based navigation techniques, of the PEN (“Performance Eased Navigation”) type. Such techniques define a coherent set of performance requirements to be met (accuracy, integrity, continuity, availability) for implementing a flight procedure without requiring the pilot and/or the co-pilot to use a specific navigation assistance means (“Sensor Based Navigation”).
A usual procedure of the RNP (“Required Navigation Performance”) type is based on such a technique. In such a case, RNP trajectories are defined, having a larger flexibility than the usual procedures based on ground beacons (VOR type or VOR/DME type procedures). Indeed, a RNP trajectory is not required to fly over beacon radio-electric coverage areas, allowing a better optimization, in particular, a minimization of the trajectory lengths. In such a case, an airplane can use any existing radio-navigation means, on the sole condition that the latter reaches the level of performance required by the RNP procedure.
Moreover, a distinction is made between RNP type operations, for which the pilot and/or the co-pilot is or are to be warned should a confinement area be left, and RNAV (“aRea NAVigation”) type operations, for which there exists an accuracy requirement on a 2RNP area, without any particular warning requirement should the confinement area be left.
A RNP type operation requires a permanent estimation of an overall error, with respect to a reference trajectory. Such an overall error or total system error TSE (“Total System Error”) corresponds to the superimposition of three components:                a NSE error for “Navigation Sensor Error”, representing an error between an estimated position and the real position of the airplane;        a FTE error for “Flight Technical Error”, representing a guiding error between the trajectory defined for guiding the airplane and that actually followed; and        a PDE error for “Path definition Error”, representing an error between the trajectory desired by the pilot and/or the co-pilot and the closest one, being available in a navigation data base.        
The PDE error is generally negligible compared to both NSE and FTE errors. Thus, those two NSE and FTE errors are particularly valuable for improving the RNP performance of an airplane.
A usual functional architecture, for implementing RNAV or RNP procedures, such as curved approaches with no ground navigation assistance, could be operated as follows:
A/ the pilot sends a command via a display and check unit of the MCDU (“Monitoring and Control Display Unit”) type, for requesting an approach. The flight management system is then called for, of the FMS (“Flight Management System”) type, which, by means of an appropriate function, looks up an internal data base for providing a list of possible approaches. The pilot selects the approach he wants amongst such a list of possible approaches, being displayed by the MCDU unit. For the selected approach, the corresponding data are sent to other systems and functions of the airplane:
B/ inertial sensors of the ADIRU type (“Air Data Inertial Reference Unit”) of a ADIRS (“Air Data and Inertial Reference System”) system send position and speed data to a position and speed calculation function of the FMS system. The calculated position is sent to a function calculating deviations with respect to the flight plane;
C/ the lateral and vertical deviations are displayed by means of systems of the EFIS (“Electronic Flight Instrument System”) type. The lateral deviation is also sent to a function for calculating the FTE component taking part in the total system error TSE. The other component of the total system error is the navigation error NSE being provided by a function issued from the ADIRS system. When the total system error TSE is calculated, the value thereof is compared to the RNP value being required, for determining the state of the RNP function;
D/ in such a usual architecture, the total system error TSE is continuously checked, and alarms are emitted if the required RNP performance is not achieved; and
E/ the guidance function implements the guidance orders for supplying an automatic pilot, if the pilot has selected an automatic pilot mode. The guidance function uses the state information and the type of selected approach for supplying a guidance mode piece of information.
The previous usual architecture has a number of limitations when it is intended to reach high levels of navigation performance. Such levels are required when low RNP operations (RNP<0.3 NM) or accuracy approaches are implemented.
Such limitations can be explained based on a calculation of the deviations. The calculation of deviations and that of the position and speed are performed in a flight managing calculator of the FMC type, being, in general, only redundant twice. The FMS system (comprising those two FMC calculators) is not breakdown proof through the simultaneous resetting of its calculators, thereby resulting in both the FMS position, the flight plane and guidance being lost, as well as the ongoing RNP operation being untimely interrupted.
Should there be an error on the position or the deviation calculation in one of the two FMC calculators of such a usual architecture, it is only possible to detect such an error. To this end, it is required that the second calculator has performed the calculation correctly and the deviation between the values provided by both FMC calculators should be checked. However, without any additional information, the pilot and/or the co-pilot do not know which of the two FMC calculators has provided the erroneous data, thereby increasing the operational load in potentially tricky flight phases (proximity of the relief if getting out of the confining area).
Furthermore, different events are feared for a low RNP value operation, in particular, the following:                regarding position: a loss of position, a loss of GNSS/GPIRS position, an undetected erroneous position;        regarding flight plane: a loss of flight plane, an undetected erroneous flight plane;        regarding guidance: a loss, a loss of automatic pilot or of the guidance calculator, an undetected erroneous guidance; and        regarding display: a loss, an undetected erroneous display.        
It should be noticed that a performance demanding operation, such as contemplated in the present invention, could also be a usual accuracy approach without any ground assistance means or a non accuracy usual approach of the FLS (“FMS Landing System”) type.
Document FR-2,887,329 is related to a low RNP flight and document FR-2,888,636 is related to a non accuracy approach.
Furthermore, it is known that the FTE guidance error closely depends on the existing delays in the whole navigation and guidance chain. Now, with the above-mentioned usual architecture, such a FTE error could be important, more specifically limiting the ability to perform low RNP value operations. Moreover, from document EP-1,464,576, a piloting assistance device is known for an aircraft upon a landing. Such a device comprises:                first calculation means for determining a position of the aircraft;        second calculation means for determining deviations between said position of the aircraft and a common segment of a flight trajectory; and        transmission means for transmitting such deviations to a guidance calculator.        